Field of the Invention
The invention relates to a gas and steam-turbine plant having a waste-heat steam generator which is connected downstream of a gas turbine on the flue-gas side and which has heating areas that are connected in a water/steam circuit of a steam turbine. The invention also relates to a method of cooling a coolant of the gas turbine of the gas and steam-turbine plant.
In a gas and steam-turbine plant, heat contained in an expanded working medium (flue gas) from the gas turbine is utilized in order to generate steam for the steam turbine. The heat transfer is effected in a waste-heat steam generator, which is connected downstream of the gas turbine on the flue-gas side and in which heating areas in the form of tubes or tube banks are disposed. The latter in turn are connected in the water/steam circuit of the steam turbine. The water/steam circuit normally includes several, for example two, pressure stages, wherein each pressure stage has a preheating heating area and an evaporator heating area.
In an evaporator circuit designated as having natural circulation, partial evaporation of a conducted working medium takes place in the evaporator. Circulation of the working medium is ensured by pressure differences which develop during the evaporation process and/or by a geodetic configuration of the evaporator and a steam drum. As an alternative, the evaporator circuit may also be a forced circulation circuit, in which the circulation of the working medium is ensured by a circulating pump and the working medium is likewise at least partly evaporated in the evaporator. In the case of both the natural circulation and the forced circulation, the water/steam mixture is fed from the evaporator to a steam drum connected in the evaporator circuit. Water and steam are separated in the steam drum, with the water being fed again from the steam drum to the evaporator.
Alternatively, in an evaporator circuit constructed as a once-through steam generator, complete evaporation of the working medium may also be provided in one pass through the evaporator with subsequent partial superheating. In that case, the passage of the working medium through the evaporator is likewise ensured by pumps.
Irrespective of whether the natural-circulation, the forced-circulation or the once-through principle is used in the steam-turbine plant, an especially high temperature of the flue gas at the inlet to the gas turbine of, for example, 1000 to 1200xc2x0 C. is the goal. That is aim at in order to increase the performance of the gas turbine and thus achieve as high an efficiency of such a gas and steam-turbine plant as possible. However, such a high turbine inlet temperature involves material problems, in particular with regard to heat resistance of the turbine blades.
An increase in the turbine inlet temperature may be permitted when the turbine blades are cooled to such an extent that they are always at a temperature which is below the admissible material temperature. To that end, it is known from European Patent 0 379 880 B to branch off a partial flow of compressed air flowing off from a compressor allocated to the gas turbine and to feed that partial flow as coolant to the gas turbine. The air which serves as the coolant is cooled before entry into the gas turbine. In the process, the heat extracted from the cooling air during the cooling is utilized as heat of evaporation and is used to drive the steam turbine. A flash evaporator having a circulating pump and a flash vessel is used as the evaporator system. In that case, water is removed from the steam drum of the evaporator circuit, that water is heated up by heat exchange with the cooling air serving as the coolant for the gas turbine and it is then evaporated by flashing in the flash vessel. The steam which is thus produced is fed to the steam turbine.
Such an apparatus for cooling the cooling air of a gas turbine is constructed for a certain temperature difference between the water removed from the steam drum and the cooling air. In that case, the operation of the entire waste-heat steam generator is necessary for reliable cooling of the gas turbine. Such a gas and steam-turbine plant can therefore only be used in a flexible manner to a limited extent.
German Patent DE 43 33 439 C1, corresponding to U.S. Pat. No. 5,661,968, discloses a gas and steam-turbine plant in which an intermediate circuit that is provided for cooling the cooling air of the gas turbine transfers heat carried in the cooling air to the water/steam circuit assigned to the steam turbine. However, in the case of that apparatus, particularly effective cooling of the cooling air is dependent on the operational readiness of the steam turbine, with the result that the flexibility of use of that plant is restricted.
It is accordingly an object of the invention to provide a gas and steam-turbine plant in which reliable cooling of the gas turbine is ensured even under various operating conditions and a method for cooling a coolant of a gas turbine of such a plant that is especially suitable for various operating conditions, which overcome the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type.
With the foregoing and other objects in view there is provided, in accordance with the invention, a gas and steam-turbine plant, comprising a gas turbine for receiving a coolant, the gas turbine having a flue-gas side; a steam turbine having a water/steam circuit; a second water/steam circuit to be selectively connected to and shut off from the water/steam circuit of the steam turbine; a waste-heat steam generator connected downstream of the flue-gas side of the gas turbine, the waste-heat steam generator having heating areas connected in the water/steam circuit of the steam turbine; and a heat exchanger for cooling the coolant of the gas turbine, the heat exchanger having a secondary side connected in the second water/steam circuit.
The invention is based on the concept that, in the case of a gas and steam-turbine plant which can be used in an especially flexible manner, the gas turbine should be operable independently of the waste-heat steam generator and thus also independently of the steam turbine. Even during solo operation of the gas turbine, reliable cooling of its is turbine blades should be ensured. To this end, a cooling system which is independent of the water/steam circuit of the steam turbine is provided for cooling the coolant of the gas turbine.
In accordance with another feature of the invention, the heat exchanger is constructed as an evaporator for the second water/steam circuit. In this case, during normal operation of the gas and steam-turbine plant, the steam obtained in the second water/steam circuit can be expediently fed to the water/steam circuit of the steam turbine.
In accordance with a further feature of the invention, there is provided a steam drum connected in the second water/steam circuit for separating the steam from the water/steam mixture flowing off from the heat exchanger.
In accordance with an added feature of the invention, the steam drum has an integrated condenser.
In accordance with an additional feature of the invention, in order to provide an especially favorable recovery of the heat extracted from the coolant of the cooling system, a steam line which can be shut off and through which the steam generated in the second water/steam circuit can be fed into a steam drum connected in the water/steam circuit of the steam turbine, is connected to the steam drum connected in the second water/steam circuit.
In accordance with yet another feature of the invention, the separate water/steam circuit can be operated by natural circulation for an especially simple type of construction and for an especially low cost in terms of assembly and installation.
In accordance with yet a further feature of the invention, the steam drum connected in the second water/steam circuit is connected to a secondary cooling circuit, so that the heat extracted from the cooling air when it is being cooled is reliably discharged even during solo operation of the gas turbine. In this case, the secondary cooling circuit may, for example, contain a cooling tower.
With the objects of the invention in view there is also provided a method for cooling a coolant of a gas turbine of a gas and steam-turbine plant, which comprises cooling the coolant by heat exchange with a medium carried in a water/steam circuit independent of the steam turbine.
In accordance with another mode of the invention, the medium carried in the independent water/steam circuit is at least partly evaporated during the heat exchange with the coolant.
In accordance with a concomitant mode of the invention, the independent water/steam circuit is operated by natural circulation.
The advantages achieved by the invention reside in particular n the fact that reliable cooling of the gas turbine irrespective of the operating state of the steam turbine is ensured by the cooling of the coolant of the gas turbine by heat exchange with a medium carried in a water/steam circuit which is independent of the steam turbine. The gas turbine can therefore be reliably operated even during various operating states of the gas and steam-turbine plant, in particular even during solo operation. In addition, due to the connection between the second water/steam circuit and the water/steam circuit of the steam turbine, which connection can be shut off, the heat extracted from the cooling air of the gas turbine when the cooling air is being cooled can be returned into the energy-generation process during normal operation of the gas and steam-turbine plant. The gas and steam-turbine plant can therefore be operated especially efficiently during normal operation.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a gas and steam turbine plant and a method for cooling a coolant of a gas turbine of such a plant, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.